Folded array CT baggage scanner

ABSTRACT

A reduced size CT scanner for baggage inspection has a wide angle x-ray source and multiple sets of detectors at different distances from the x-ray source. The detectors in each set are sized and positioned to maintain consistent pitch and flux levels among all detectors. Conventional reconstruction processes can be used to process the data from the CT scanner. The scanner may also be incorporated into a check-in desk in a network of scanners.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a Computed Tomography (CT) system forinspecting baggage for explosives or other contraband. Moreparticularly, it relates to a novel Computed Tomography (CT) scannerdesign having a reduced size.

2. Discussion of Related Art

Following the terrorist attacks on Sep. 11, 2001, the United Statesgovernment decided to implement additional airport security. One of thesecurity measures which were to be implemented was inspection of allchecked baggage for explosives. In November 2001, Congress passed theAviation and Transportation Security Act, which mandated 100% explosivedetection screening of checked baggage by Dec. 31, 2002. It was expectedthat the screening requirements would be met through a combination ofExplosive Detection Systems (EDS) and Explosive Trace Detection systems(ETD). EDS is faster, but more expensive and more prone to false alarms.ETD is much slower, more invasive and requires more manual input.Congress later extended the deadline for full deployment until Dec. 21,2003, on a case by case basis, as long as approved interim methods, suchas canine teams, hand searches and passenger bag matching, wereemployed.

Implementation of this security measure using existing technology hasbeen and will continue to be cumbersome and expensive. Even by the endof 2003, the implemented solutions are unlikely to be permanentsolutions. In many cases, airports have deployed hand-fed machines interminal lobbies, usurping premium space, or implemented themanpower-intensive ETD systems. In order to improve efficiency, reducemanpower requirements, and recover lobby space, airports will transitionto EDS machines integrated into airport baggage handling system.However, existing EDS machines are not easily deployed or integratedinto existing baggage handling systems.

Known Explosives Detection Systems (EDS) utilize either ComputedTomography (CT) technology, or a combination of x-ray and CT technology,to create an image of the contents of a bag. Projection x-ray systemshave been used for many years with carry-on baggage. However, suchsystems require operator review of all images and provide slowthroughput. Furthermore, these systems also cannot provide thickness ordensity information for objects in order to provide explosive detection.

A Computed Tomography (CT) machine has been designed to performautomated explosives detection for passenger baggage, parcels, mail, andsmall cargo prior to loading onto an aircraft. CT technology has beenproven to successfully meet the US TSA Certification requirements forautomated explosives detection (EDS) in airline checked baggage. WhileCT technology is effective for explosive detection, use of existing CTtechnology in checked baggage inspection has many drawbacks. CT machinesincorporate a rotating ring or “gantry” on which the X-ray source anddetectors are mounted. FIG. 1 is a cross sectional view of aconventional CT scanner 10. The CT scanner 10 includes a gantry 11surrounding a tunnel 20. A conveyor (not shown) moves baggage throughthe tunnel 20 for scanning. The gantry 11 rotates about the tunnel,producing one slice of data for each rotation. An x-ray source 30produces a narrow angle beam 40. A detector 31 is positioned on thegantry 11 to intersect the x-ray beam 40 passing through the tunnel. Thedetector 31 may consist of multiple detectors which are located equaldistances from the x-ray source. The x-ray source 30 and detector 31must be sized and positioned so that the entire region to be inspectedfalls within the x-ray beam. The data from the detector is analyzedusing a computer to generate a cross sectional image orthree-dimensional representation of the contents of the baggage beingscanned.

These CT based systems have many drawbacks. They are large, heavy andrequire substantial space. The tunnel 20 has to be large enough toaccommodate substantially all sizes of checked baggage. The x-source 30and detector 31 must be positioned sufficiently distant from the tunnel20 so that the entire tunnel is within the x-ray beam 40. Furthermore,the gantry 11 has to be large enough to accommodate the entire detector31, positioned equidistant from the source. The x-ray source 30 alsomust be powerful enough to provide sufficient flux at the detector 31,after passing through the baggage, to overcome noise and allow foranalysis. The gantry 11 also must be sturdy enough to support andbalance the large x-ray source for high-speed rotation. In typical CTsystems, the gantry 11 is 5/3 to 2 times the size of the tunnel 20.Furthermore, the system requires significant shielding, generally lead,to protect the operators and passengers from exposure to the powerfulx-rays.

Implementation of known EDS systems for review of all baggage has provento be difficult. In many cases, the EDS system is installed in the lobbyof the airport, where it occupies a great deal of space otherwiseavailable to passengers, retails stores, and other security processes.The large weight of the system often requires strengthened lobby floors.Alternatively, as has been done in some airports, the EDS system couldbe integrated into the baggage conveyor system which transfers thebaggage from the lobby to the loading area. However, such systems costmillions of dollars in infrastructure modifications and disruption ofairport operations to install. Also, the conveyors through the CT systemmust move at a slow speed to generate sufficient data for reconstructionof the contents of the baggage from the slices. In order to accommodatethe volume of baggage at a large airport, a substantial number ofscanners are required. With multiple scanners, coordination of operationissues arise.

SUMMARY OF THE INVENTION

The deficiencies of the prior art are substantially overcome by the CTscanner of the present invention which includes a wide angle x-ray tubeand multiple sets of detectors. One set of detectors is centered on thex-ray source with one or more additional sets covering outer edges ofthe wide angle x-ray beam. The CT scanner of the present invention has areduced size while maintaining the same tunnel size as conventionalscanners. According to another aspect of the invention, the sets ofdetectors have different pitches in order to maintain a constantinter-detector angle. Thus, reconstruction algorithms can be simplified.According to another aspect of the invention, detectors are positionedon only half of the detector ring. According to another aspect of theinvention, a second set of detectors sensitive to a different energyspectrum are positioned on the other half of the detector ring. The CTscanner of the present invention can provide dual energy analysis from asingle scan of a bag, using the multiple sets of detectors.

According to another aspect of the invention, the x-ray source operatesat a reduced flux level from conventional systems. Due to the compactsize of the CT scanner of the invention, the shorter distances from thesource to the detector allows for a less powerful x-ray. According toanother aspect of the invention, a portion of a bag can be scannedmultiple times with the results being averaged. The data from themultiple scans is averaged to remove noise. The resulting reduction innoise allows a reconstruction of heavy bags with a reduced flux x-raysource.

According to another aspect of the invention, the reduced size CTscanner is incorporated into an airline check-in desk. The CT scanner ispositioned to allow the passenger to insert the bag to be checked. Uponsuccessful scanning and analysis (or prior to scanning and analysis),the baggage is tagged by airline personnel in the regular manner andtransferred to the existing baggage handling system. According toanother aspect of the invention, a plurality of CT scanners arenetworked

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view of a conventional CT scanner.

FIG. 2 is a perspective view of a CT scanner according to an embodimentof the present invention.

FIGS. 3A and 3B are perspective views of a gantry system according to anembodiment of the present invention.

FIG. 4 is a cross sectional illustration of a CT scanner gantryaccording to an embodiment of the present invention.

FIG. 5 is a perspective view of a detector array component of a CTscanner according to an embodiment of the present invention.

FIG. 6 is a front view of a portion of a detector of a CT scanneraccording to an embodiment of the present invention.

FIG. 7 is a cross sectional view of a detector component of a dualenergy CT scanner according to an embodiment of the present invention.

FIG. 8 is a block diagram of the components of a CT scanner according toan embodiment of the present invention.

FIG. 9 is a block diagram of computer components of a CT scanneraccording to an embodiment of the present invention.

FIG. 10 is a perspective view of an airline check-in desk according toan embodiment of the present invention.

FIG. 11 is a block diagram of a networked CT scanner system according toan embodiment of the present invention.

FIG. 12 represents the pitch between detector elements according to anembodiment of the invention.

FIG. 13 is a cross sectional view of a CT scanner according to anotherembodiment of the present invention.

FIG. 14A-C represents data from the CT scanner of FIG. 13.

FIG. 15 is a cross sectional view of a CT scanner according to anotherembodiment of the present invention.

DETAILED DESCRIPTION

A CT scanner of the present invention has a more compact size thanconventional scanners through the use of a wide angle x-ray source and afolded detector array, including sets of detector arrays at differentdistances from the x-ray source. FIG. 2 illustrates an embodiment of aCT scanner 100 according to an embodiment of the present invention. TheCT scanner 100 includes a housing 110 with a substantially circulartunnel 120, there through. The tunnel 120 has an input end 121 and anoutput end 122. A conveyor 123 extends from the input end 121 to theoutput end 122 of the tunnel 120. For ease loading baggage, the conveyormay extend beyond the ends of the tunnel 120. Additionally, otherconveyors may be positioned and used to transfer baggage to or from theconveyor 123 in the CT scanner 100. Coverings (not shown), such as leadlined rubber or fabric, may be placed within the tunnel 120 or at theinput end 121 and output end 122, to provide x-ray shielding. The CTscanner 100 has a x-ray area 130 towards its center. As illustrated inFIG. 2, the x-ray area 130 may be larger than the ends 121, 122 of thetunnel 130. Alternatively, the housing 110 could be formed of a singlesize to accommodate the size of the x-ray area 130.

FIGS. 3A and 3B illustrate a gantry system disposed within the x-rayarea 130 of the housing 110. The gantry system includes a supportstructure 210 (FIG. 3A) and a gantry 220 (FIG. 3B). The supportstructure 210 includes a base 211 and a circular shaped vertical support212. The vertical support 212 is attached to the base 211. The gantry220 includes a ring 221 which is rotatably attached to the verticalsupport 212 so that the tunnel 120 passes through the center of the ring221. An x-ray source 230 is attached to the ring 221 to provide a wideangle x-ray beam over the entire area of the tunnel 120. Preferably, thex-ray source has a rounded shape on an upper surface, similar in shapeto the outer surface of the gantry. In this manner, the size of thegantry can be reduced since it does not need to accommodate theextensions of a rectangular x-ray source. Furthermore, the gantry has athree sets of detector arrays 240, 241, 242, positioned on the ring 221within the projected beam of the x-ray source 230. Notably absent fromthe gantry is a computer. Conventional CT scanners include a computer onthe gantry to process data from the detectors and format the data fortransmission from the gantry to a host computer which operates thescanner. According to an embodiment of the present invention, the CTscanner does not include a computer on the gantry. Instead, data fromthe detectors are streamed directly to the host computer. Elimination ofthe computer on the gantry permits a smaller and lighter gantry. As withconventional CT scanners, the gantry includes an edge contact (notshown) for transferring power and control signals to the gantry and datafrom the gantry while the gantry is rotating. Of course, the presentinvention may include a computer on the gantry to perform some of theprocessing of data as in conventional CT scanners.

FIG. 4 illustrates the positioning of the x-ray source 230 and thefolded array detectors 240, 241, 242 on the ring 221. The ring 221 ofthe gantry 220 is substantially smaller than for conventional CTscanners with a similar size tunnel 120. According to an embodiment ofthe present invention, the gantry has a diameter of approximately 130cm, when the tunnel is approximately 80 cm in diameter. A conventionalCT scanner with a 80 cm diameter tunnel would be approximately 210centimeters. Thus, with the present invention, the CT scanner is reducedin size by approximately 40%. The wide angle x-ray source 230 is locatedon the gantry 220 so that the x-ray beam intersects the entire area ofthe tunnel 120. Since the x-ray source 230 has a wider beam 250 than fora conventional CT scanner, it can be positioned closer to the tunnel.The CT scanner of the present invention includes a folded detector arrayincluding two or more sets of detectors 240, 241, 242 at differentdistances from the x-ray source 230. One set of detectors 240 covers acentral portion of the gantry ring and is centered on the x-ray sourceat a distance equal to the source to center detector distance. The othersets of detectors 241, 242 are positioned to intersect the outer rangesof the x-ray fan beam 250. According to an embodiment of the invention,the second sets of detectors 241, 242 start at the point in the beam 250where the inner detectors 240 reach the edge of the ring 221.Alternatively, the second set of detectors may start at any location inthe beam 250 where the inner detectors end, depending upon the size ofthe machine and the gantry. The second set of detectors 241, 242 are acloser distance to the x-ray source 230 than the inner detectors 240.

Since the detectors are closer to the x-ray source, they can have ashorter pitch than for conventional CT scanners without sacrificing fluxlevels or photon counts. According to an embodiment of the presentinvention, the detector elements are sized and positioned so that eachset of detectors has substantially identical pitch and flux levels, Thecloser distance between the x-ray source 230 and the second sets ofdetectors 241; 242 allows a smaller detector pitch than for the firstset of detectors 240. Since the distance is shorter, the photon countper unit area at the second sets of detectors 241, 242 is higher thanfor the first set of detectors 240. The higher photon count allows areduced detector pitch without sacrificing the signal to noise ratio.Additionally, with the reduced pitch, a constant inter-detector anglecan be maintained throughout the fan beam 250. FIG. 12 illustrates therelationship between the first set of detectors 240 and the second setsof detectors 241, 242. The angles between detectors (α) remains constantwhile the pitch between detectors (δ1, δ2) changes. To maintain the sameangle, the equation δ1/R1=δ2/R2 must be satisfied. Additionally, tomaintain a minimum flux level, the following equation, (δ2×w2)/R2²≧(δ1×w1)/R1 ², where w1 and w2 are the widths of the detectors, must besatisfied. Since the distance is reduced, both equations can besatisfied, with consistent angles and minimal noise interference. Thesefeatures allow for simplified reconstruction software. Known‘equi-angular’ fan beam reconstruction algorithms can be used withminimal modification. According to an embodiment of the invention, thedetector elements of the first set of detectors 240 are approximately 10mm long and 2.2 mm wide; The detector elements of the second sets ofdetectors 241, 242 are 8 mm long and 1.8 mm wide.

FIGS. 5 and 6 illustrate detector assemblies for simplifying theassembly of the CT scanner of the present invention and to improvequality control. FIG. 5 illustrates a detector assembly 300 having ahousing 310, a processing board 320 and a detector array 330. Thedetector array 330 includes a plurality of detectors (not shown)arranged along the width of the assembly 300. As illustrated in FIG. 6,detector assemblies 300, 301, 302 are attached to the ring 221 of thegantry 220. The detector assemblies 300, 301, 302 are positioned so thatthe detector or each end of the detector array 330 is adjacent to adetector on the end of an adjacent detector array. Ideally, for purposesof reconstruction, every detector in the array would be perpendicular toand equidistant from the x-ray source. However, with flat detectorassemblies 300 there are slight variations in incidence angle anddistance across the assembly. According to an embodiment of theinvention, a detector assembly at the center of the first set ofdetectors 240 is arranged with a center detector element beingperpendicular to the position of the x-ray source. For the remainingdetector assemblies, the outermost detector element is perpendicular tothe x-ray source. As illustrated in FIG. 6, the x-ray source direction350, 351 is perpendicular to the left hand side of an assembly on theleft hand side of the gantry. The right hand side of assemblies on theother side of the gantry would be positioned perpendicular to thedirection of the x-ray source. Such positioning allows the detectorarrays to be properly angled and nested, as illustrated in FIG. 6 tominimize the distance between adjacent elements on different assemblies.

The CT scanner of the present invention is smaller and less costly thanconventional scanners. The wide beam width provides additional benefitsin addition to reduced diameter. The shorter source to detectordistances, allow for less powerful x-ray sources which translates tolower part costs, simplified cooling (quieter) and power requirements.The decrease in x-ray flux also requires less shielding, againtranslating to a cheaper (and lighter) system.

According to an embodiment of the present invention, the CT scanneroperates in a dual energy mode. FIG. 7 is a cross sectional view of adetector element 330 for dual energy operation. The detector elementincludes a low energy scintillator layer 331, a low energy photodiodelayer 332, a copper layer 333, a high energy scintillator layer 334, anda high energy photodiode layer 335. The high energy layers 334, 335 arewider than the low energy layers 331, 332. According to an embodiment ofthe invention, the low energry layers are approximately 5 mm long andthe high energy layers are approximately 10 mm long. The differentlengths create similar flux levels between the low and high energylayers, even with the greater shielding from the copper and additionallayers, thereby simplifying data acquisition electronics and subsequentsignal processing. Alternatively, a dual energy scan can be performedusing known techniques with a pulsing x-ray source and a singlephotodiode layer in the detectors.

FIG. 8 illustrates the components of a CT scanner 100 according to anembodiment of the present inventionThe CT scanner 100 includes thegantry 221, and two computers 400, 500. A host computer 400 controlsoperation of the scanner and retrieves data from the detectors. Adetection algorithm computer 500 operates on the data to determinewhether an explosive device or other object of interest is present. Ofcourse, a single computer could be used to perform all of the functionsfor the CT scanner. However, the use of two computers prevents theextensive processing of the detection algorithm from slowing down theoperation and data collection of the CT scanner. Also, control and dataelements are connected between the gantry 221 and the computers 400,500. An AC power input 224, connected to ordinary 240 V AC power,provides the power for the CT scanner. A DC power supply 225 receivesthe AC power and converts it to DC power for powering the processingelements on the gantry. A set of motor drives 222, powered by the ACpower, or alternatively by the DC power, operate the conveyor and rotatethe gantry. A data link 223 connects the detector assemblies to the hostcomputer 400. DC power and the data link are positioned on the ring ofthe gantry to provide data during rotation. The circuit boards 320 onthe detector assemblies 300 sample the detectors 1440 times per second.The data is then transferred, through the data link 223 to the hostcomputer 400. Encoders are also used to determine the rotationalposition of the gantry and of the conveyor. This data is also providedto the host computer.

The components of the computers are illustrated in FIG. 9. The hostcomputer 400 includes a motherboard 410 and a data acquisition card 420.The data acquisition card 420 includes inputs from the imaging array423, the conveyor belt encoder 422, and the gantry encoder. 421. It alsoincludes a field programmable gate array card 424 for retrieving thedata and forwarding it to the motherboard 410. The motherboard 410includes a processor 411, such as a Pentium processor from Intel, alarge RAM 412, and a back-projection accelerator card 520 for processingthe data. It also includes a Host/PCI bridge for sending and receivinginformation from the data acquisition card and other computers. The dataretrieved from the CT scanner is transferred to the detection algorithmcomputer 500. An ethernet connection allows quick transfer of the largeamounts of data. The detection algorithm computer also includes amotherboard 510 for proper processing of the data to determine theexistence of explosives or other materials.

The data from the CT scanner having a folded detector array can beprocessed using conventional CT operating and reconstruction techniques,such as helical scanning or start/stop scanning. The host computer isprogrammed to control the x-ray source, conveyor belt, and reading ofdetectors according to the desired scanning scheme. The detectionalgorithm computer, similarly, is programmed to reconstruct the CT databased upon the scanning scheme which is utilized.

The smaller size of the CT scanner of the present invention allows it tobe more easily incorporated into the baggage handling process thanexisting EDS systems. FIG. 10 illustrates an airline check-in desk 700incorporating a baggage scanner according to an embodiment of thepresent invention. As in some conventional check-in desks, the desk 700of the present invention includes two check-in stations 710, 720 with achecked baggage loading area 730 between them. Multiple desks 700 couldbe positioned together in the airport lobby. The CT scanner 100 of thepresent invention is positioned at the baggage loading area 730. Apassenger, upon checking in, places his or her baggage on the conveyorat the baggage loading area 730 or as a standalone system. The baggageis automatically scanned and processed for the existence of explosives.If the baggage passes, the check-in personnel can tag it in the ordinarymanner and forward it to the baggage handling system. With the check-indesk of this embodiment of the present invention, very little additionallobby space is needed for scanning baggage, and no modifications to thebaggage handling system are required. As an option, the bag can beprocessed as usual, with the bag tag placed by the ticket agent prior toscanning. Once checked-in, the bag is scanned and injected into thebaggage handling system. If desired for improved operational flow, anysuspect bag can be resolved in the baggage makeup area away from thepassengers in the terminal area. Alternatively, the CT scanner of thepresent invention may be incorporated into a self service check-in desk,either as part of a check-in area of the lobby or as a free standingkiosk.

As described above, the CT scanner of the present invention may be afree standing, standalone unit with a TSA operator available forreviewing images and addreesing potential threats. According to anotherembodiment of the present invention, the host computers of a pluralityof CT scanners 700 are networked together to form a single EDS. Thenetworked CT scanners may include any combination of free standingunits, self check-in units and integrated check-in desk units. Most bagscan be automatically cleared by analysis of the scan data. However, somemay require further review by an operator. Rather than have an operatorat each scanner, as is used with carry-on baggage, a threat control room760 with a plurality of operators is connected to the network. If a bagcannot be cleared automatically, the projection and/or reconstructiondata is transferred to an operator in the threat control room 760. Theoperator can then determine whether a threat exists or can be cleared.If the possible threat is cleared by the operator, the bag is passed tothe baggage handling system in the ordinary manner. However, if thethreat cannot be cleared, the bag would be passed to other operators fora manual hand search. The network may include additional equipment, suchas network server computers 750, printers 771, network control stations772, and remote resolution computers 773.

The small size of the baggage scanning system also allows it to beplaced in other locations for distributed handling of checked baggage.For example, a scanner could be placed for curbside check-in or atrental car return areas for simplified processing. Additionally, hotelsor other locations could provide check-in baggage service by providing ascanner and a secure area for scanned baggage. Guests could have theirluggage automatically processed by the hotel and securely transferred tothe airport without the need for further processing or delay duringcheck-in. Additionally, the size of the scanner could permit it to beplaced in a vehicle used to pick up baggage from various locations fortransport to the airport. Free standing units can be provided at variouslocations in the airport for either self-check-in or assisted check-inof passengers. Alternatively, a CT scanner of the present invention maybe used at security checkpoints for scanning of carry-on baggage. Eachscanner, independent of its location in the airport or away from theairport, may be operated as a stand alone unit or may be networked forcommon review by TSA operators.

FIG. 13 illustrates another embodiment of the present invention whichprovides additional reductions in the size and cost. In the thirdembodiment, the CT scanner 810 has multiple detectors 841, 842, 843,844, 845 located on half of the detector ring 811. As in the first twoembodiments, the detectors are at different distances from the x-raysource 330 and at different angles. Using half a detector ring issufficient for creating a CT reconstruction with a full 360 degrees ofdata. The half of a detector ring is mathematically equivalent tocollecting data for 180 degrees plus fan beam, which is required forreconstruction. FIGS. 14A-14C illustrate CT reconstruction. FIG. 14Aillustrates reconstruction using a full detector ring. FIG. 14Billustrates reconstruction of the same data from half a detector ring.The data from FIG. 14B can be used to complete a full reconstruction asillustrated in FIG. 14C. Using only half a detector ring, as in thisembodiment, allows the imaging cost to be substantially reduced becausefewer detectors are required.

In order to cut the cost further, the present invention may operate withreduced 110 flux levels from conventional CT scanners. In the design ofx-ray systems for baggage, one often designs the flux to be enough topenetrate the heaviest 5% of the bags. The majority of the bags requiresignificantly less flux. By designing the system to provide flux foronly the 90^(th) percentile bags, the amount of flux is significantlyreduced. The reduced flux allows reduction in the noise levels andrequired shielding, which substantially reduces size and cost of thedevice. In order to handle the heavier bags, two or more scans areperformed on the bags and the sinograms are averaged. The raw data canbe analyzed, as discussed below, to determine which bags or regions of abag are likely to require the extra flux. The bag slice gets scannedtwice or more, averaging the second set of raw data with that obtainedin the first rotation, which will reduce the noise in the reconstructedimage and mimics a higher flux system. One can do this a 3rd or 4th timefor those 1 thousandth bag slices of very heavily cluttered bags. Thepresent invention reduces the need for manual bag searches by providinga practical solution to the “exceptional” or “shield” bag problem. Heavyor “shield” objects in unusual bags are scanned several times to obtainuseful data with respect to heavily cluttered areas, without the needfor a manual search.

According to another embodiment of the present invention, the CT scannerof the present invention can be used for dual energy CT scans with asingle pass of the baggage through the scanner. FIG. 15 illustrates a CTscanner 910 according to this embodiment of the invention. The CTscanner 910 includes a wide-angle x-ray source 930 and two sets ofdetectors 940, 950. Each of the sets of detectors 940, 950 includes aplurality of detector elements 941, 942, 943, 944, 945, 951, 952, 953,954, 955. The detector elements for each set are each positioned on onehalf of the detector ring 911, as in the third embodiment. The two setsof detectors 940, 950 are sensitive to different energy spectrums. Usingthe CT scanner of this embodiment, two CT slices, of different energies,can be obtained from a single rotation of the detector ring. This willallow for a dual energy analysis of the CT images. One can also obtaindual energy CT images with back-to-back detectors. The dual energy CT isvery useful to reduce the false alarm of the machine. This is verydesirable in an airport environment. Although FIG. 15 illustrates fivedetector arrays for each set of detectors, any number of detector arraysmay be used. Each set of detectors 940, 950, corresopnding to an energylevel of the x-ray source, is positioned on one half of the detectorring 911.

The present invention has been described with respect to inspection ofchecked baggage, since it has substantial uses in that field. However,the reduced size of the folded array CT scanner according to the presentinvention can be used in many other applications for which CT scannersare used. For example, it may be used for cargo or pallette inspectionor screening. A larger tunnel is typically necessary for cargo andpallette inspection, and conventional CT scanners are correspondinglymuch larger. The CT scanner of the present invention has a significantlysmaller size, even with a large tunnel. The present invention may alsobe used in medical CT scanning applications.

Furthermore, the present invention has been described as including anx-ray source. Conventionally, CT scanners have used x-ray sources forradiation and appropriate detectors for x-rays. Of course, otherradiation sources, such as gamma radiation, could also be used inconnection with the present invention. With a different radiationsource, the detectors and/or scintillator material would need to bechanged as appropriate to measure the proper radiation spectrum.

Having described at least one embodiment of the invention,modifications, adaptations and improvements will be readily apparent tothose of ordinary skill in the art. Such modification, changes andadaptations are considered part of the invention.

1. A detector array adapted for use in a computed tomography scanner,the detector: array comprising: a gantry having an opening; a detectorarray, comprising: a first set of detectors mounted to the gantry, thefirst set of detectors comprising a plurality of adjacent detectorelements, the first set of detectors having a first end; and a secondset of detectors mounted to the gantry, the second set of detectorscomprising a plurality of adjacent detector elements, the second set ofdetectors having a second end, wherein the second end is adjacent thefirst end and the second set of detectors is mounted with the second endoffset from the first end, whereby a gap is formed between the first setof detectors and the second set of detectors.
 2. The detector array ofclaim 1, wherein the second end is offset from the first end in adirection towards the opening.
 3. The detector array of claim 1, mountedin computed tomography scanner comprising a tunnel having an axis, withthe gantry movably mounted to provide motion about the axis.
 4. Thedetector array of claim 1, additionally comprising a third set ofdetectors mounted to the gantry, the third set of detectors comprising aplurality of adjacent detector elements, the third set of detectorshaving a third end, and wherein the first set of detectors comprises afourth end, and the fourth end is adjacent the third end and the thirdset of detectors is mounted with the third end offset from the fourthend, whereby a second gap is formed between the first set of detectorsand the second set of detectors.
 5. The detector array of claim 4,wherein the second end is offset from the first end in a directiontowards the opening and the third end is offset from the fourth end in adirection towards the opening.
 6. The detector array of claim 4, incombination with a radiation source positioned to direct radiation atthe opening to form a computed tomography scanner.
 7. The computedtomography scanner of claim 6, wherein the radiation source is awide-angle radiation source.
 8. The computed tomography system of claim6, wherein the first set of detectors is centered a first distance fromthe radiation source, the second set of detectors is centered a seconddistance from the radiation source, and the third set of detectors iscentered a third distance from the radiation source and the firstdistance is different than the second distance and the third distance.9. The computed tomography scanner of claim 8, wherein the computedtomography scanner is sized for installation in an airport passengercheck-in counter.
 10. A detector array adapted for use in a computedtomography scanner, the detector array comprising: a gantry having anopening with an axis passing therethrough; a first set of detectorsmounted to the gantry, the first set subtending a first angular rangearound the gantry, the first set of detectors having a first end and asecond end; and a second set of detectors mounted to the gantry, thesecond set subtending a second angular range around the gantry, thesecond set of detectors having a third end and a fourth end, the secondangular range being distinct from the first angular range and the firstend being spaced from the third end in a direction transverse to theaxis.
 11. The computed tomography scanner of claim 10, additionallycomprising a third set of detectors subtending a third angular rangearound the gantry, the third angular range being distinct from the firstangular range and the second angular range.
 12. The computed tomographyscanner of claim 11, wherein the third set of detectors has a fifth endand a sixth end, and the second end is spaced from the fifth end in adirection transverse to the axis.
 13. The computed tomography scanner ofclaim 11, wherein the first set of detectors, the second set ofdetectors and the third set of detectors comprise portions of a singledetector array.
 14. The computed tomography system of claim 13, whereinthe first set of detectors is centered a first distance from theradiation source, the second set of detectors is centered a seconddistance from the radiation source, and the third set of detectors iscentered a third distance from the radiation source the first distancebeing different than the second distance and the third distance.
 15. Anexplosive detection system for screening airport passenger baggage,comprising: a computed tomography scanner comprising: an x-ray source; agantry having an opening therethrough; and a detector array disposed onthe gantry, comprising: a first sub-array having a first end and asecond end with the first end positioned a first distance from theopening; and a second sub-array having a third end and a fourth end,with the third end positioned a third distance from the opening and thefourth end positioned a fourth distance, greater than the third distancefrom the opening, and wherein the first end is adjacent the third endand the first distance is greater than the third distance whereby a gapis formed between first sub-array and the second sub-array.
 16. Theexplosive detection system of claim 15, wherein the first end is offsetfrom the third end in a direction away from the opening.
 17. Theexplosive detection system of claim 15, wherein the first sub-array andthe second sub-array each comprises a plurality of adjacent detectors.18. The explosive detection system of claim 15, wherein the firstsub-array and the second sub-array are not equidistant from the x-raysource.
 19. The explosive detection system of claim 15, additionallycomprising: a third sub-array having a fifth end and a sixth end, withthe fifth end positioned a fifth distance from the opening and the sixthend positioned a sixth distance, greater than the fifth distance fromthe opening, and wherein the fifth end is adjacent the second end andthe second distance is greater than the fifth distance.
 20. Theexplosive detection system of claim 19, wherein the first sub-array iscentered on a first line bisecting the gantry into a first portion and asecond portion and the second sub-array is contained within the firstportion and the third sub-array is contained within the second portion.